Optoelectronic transmitter integrated circuit and method of fabricating the same using selective growth process

ABSTRACT

Provided are an optoelectronic (OE) transmitter integrated circuit (IC) and method of fabricating the same using a selective growth process. In the OE transmitter IC, a driving circuit, which includes a double heterojunction bipolar transistor (DHBT) and amplifies received electric signals to drive an electroabsorption (EA) modulator, and the EA modulator with a multi-quantum well (MQW) absorption layer are integrated as a single chip on a semi-insulating substrate. The MQW absorption layer of the EA modulator and an MQW insertion layer of the DHBT are formed to different thicknesses from each other using a selective MOCVD growth process.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 2004-95949, filed Nov. 22, 2004, the disclosure of whichis incorporated herein by reference in its entirety.

BACKGROUND

1. Field of the Invention

The present invention relates to an optoelectronic transmitterintegrated circuit and method of fabricating the same and, morespecifically, to an optoelectronic transmitter integrated circuit havingan integrated single chip structure in which an absorption layer of anelectroabsorption (EA) modulator and a quantum well (QW) layer of adouble heterojunction bipolar transistor (DHBT) have differentthicknesses, and method of fabricating the same using a selective MOCVD(Metal Organic Chemical Vapor Deposition) growth method.

2. Discussion of Related Art

A conventional long-wavelength optical transmitter integrated circuit inwhich a semiconductor laser and a DHBT are integrated is widely used inoptical communication systems. In general, such an optical transmitterintegrated circuit is fabricated by growing the semiconductor laserstructure on a substrate and then growing the DHBT structure on thesubstrate on which the semiconductor laser is grown.

In the conventional optical transmitter integrated circuit in which thesemiconductor laser is integrated, when a driving current is supplied toan active layer to drive the semiconductor laser, the refractive indexof the active layer is changed so that oscillation wavelength fluctuates(this is generally termed “chirping”) and thus, modulation speed islimited. For this reason, the conventional optical transmitterintegrated circuit cannot be readily utilized for high-speed andlong-distance optical communications.

In another case, the conventional optical transmitter integrated circuitmay include an electroabsorption (EA) modulator in place of asemiconductor laser. In this case, when a reverse bias voltage isapplied to a quantum well absorption layer, a quantum confined starkeffect (QCSE) occurs so that the absorption band wavelength of light ischanged. That is, because no chirping takes place unlike in thesemiconductor laser, the optical transmitter integrated circuit in whichthe EA modulator is integrated can be used for high-capacitylong-distance optical communications. However, since a conventional pintype EA modulator has the same epitaxial crystal structure as a DHBTconsisting of p⁺-base, i-collector and n⁺-sub collector, it cannot havean independent epitaxial structure and optimize its characteristics.Thus, it is difficult to further improve the modulation efficiency anddriving characteristic of the optical transmitter integrated circuit(IC).

SUMMARY OF THE INVENTION

The present invention is directed to an optoelectronic (OE) transmitterintegrated circuit (IC) and method of fabricating the same, in which amulti-quantum well (MQW) absorption layer of an electroabsorption (EA)modulator is formed to a thicker thickness using a selective MOCVDgrowth process so as to enhance the coupling and modulation efficiencyof the EA modulator. Also, the EA modulator is integrated as a singlechip on a semi-insulating substrate along with a driving circuitfabricated using the DHBT, which amplifies small electric signals to thehigh gained large electrical signal, so that the fabrication of the OEtransmitter integrated circuit is simple.

One aspect of the present invention is to provide an OE transmitterintegrated circuit including: a semi-insulating substrate; an EAmodulator disposed in a first region on the semi-insulating substrateand having a first sub-collector layer, a first clad layer, an MQWabsorption layer, and a second clad layer; and a DHBT disposed in asecond region adjacent to the first region on the semi-insulatingsubstrate and having a second sub-collector layer, a collector layer, anMQW insertion layer, a base layer, an emitter layer, and an ohmic layer,wherein the MQW insertion has a smaller thickness than that of the MQWabsorption layer.

The EA modulator may further include a first electrode disposed on apredetermined region of the second layer; and a second electrodedisposed on a predetermined region of the first sub-collector layer.

The DHBT may further include an emitter electrode disposed on apredetermined region of the ohmic layer and having a mesa structure; abase electrode disposed on a predetermined region of the base layer; anda collector electrode disposed on a predetermined regions of the secondsub-collector layer.

Another aspect of the present invention is provide a method offabricating an OE transmitter integrated circuit including steps of:forming a sub-collector layer and a first semiconductor layer on asemi-insulating substrate; forming insulating patterns on the firstsemiconductor layer, the insulating patterns required for performing aselective metal organic chemical vapor deposition (MOCVD) process;forming a relatively thick MQW absorption layer in a first region of theexposed first semiconductor layer between the insulating patterns, andforming a relatively thin MQW insertion layer in a second region of theexposed first semiconductor layer outside the first region, byselectively growing an MQW active layer in the first and second regions;removing the insulating patterns and forming a second semiconductorlayer, a third semiconductor layer, and an ohmic layer on the exposedsemi-insulating substrate; isolating the first region from the secondregion; forming a DHBT in the second region, the DHBT having thesub-collector layer, a collector layer formed of the first semiconductorlayer, the MQW insertion layer, a base layer formed of the secondsemiconductor layer, an emitter layer formed of the third semiconductorlayer, and the ohmic layer, which are sequentially stacked; and formingan EA modulator in the first region, the EA modulator including thesub-collector layer, a first clad layer formed of the firstsemiconductor layer, the MQW absorption layer, and a second clad layerformed of the second semiconductor layer.

The step of forming the DHBT may include steps of: forming a mesa typeemitter by selectively etching the ohmic layer and the emitter layer inthe second region, and forming an emitter electrode on the ohmic layer;forming a base electrode on a predetermined region of the base layer;and selectively removing top portions of the base layer, the insertionlayer, the collector layer, and the sub-collector layer outside the baseelectrode and forming a collector electrode on a predetermined region ofthe exposed sub-collector layer.

The step of forming the EA modulator may include steps of: removing theohmic layer and the third semiconductor layer from the first region;defining a waveguide type EA modulator by removing top portions of thesecond clad layer, the MQW absorption layer, the first clad layer, andthe sub-collector layer from the first region; and forming a secondelectrode on a predetermined region of the second clad layer and forminga first electrode on a predetermined region of the exposed sub-collectorlayer.

In order to have thicker crystal thickness the gap distance between theinsulating patterns for forming the MQW absorption layer in the firstregion may be smaller than a distance between the insulating patternsfor forming the MQW insertion layer in the second region, if any.

The MQW active layer may be formed of MQW i-InGaAs—InP and formed bygrowing on the first semiconductor layer formed of InP using a selectiveMOCVD process.

The step of isolating the first region from the second region mayinclude a step of; removing the ohmic layer, the third semiconductorlayer, the second semiconductor layer, the MQW active layer, the firstsemiconductor layer, the sub-collector layer, and a predeterminedportion of the semi-insulating substrate between the first and secondregions.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent to those of ordinary skill in the art bydescribing in detail exemplary embodiments thereof with reference to theattached drawings in which:

FIG. 1 is a cross sectional view of an optoelectronic (OE) transmitterintegrated circuit according to an exemplary embodiment of the presentinvention;

FIGS. 2 thru 2B are cross sectional views of a substrate before andafter a selective MOCVD growth process is performed, respectively;

FIG. 3 is a cross sectional view of a substrate on which a selectivegrowth process and a final growth process are performed; and

FIGS. 4A through 4F are cross sectional views illustrating a method offabricating an OE transmitter integrated circuit according to anexemplary embodiment of the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which exemplary embodimentsof the invention are shown. In the embodiments of the present invention,an electroabsorption (EA) modulator formed of n-InP/multi-quantum well(MQW) i-InGaAs—InP/p⁺InGaAs and a double heterojunction bipolartransistor (DHBT) formed of n-InP/MQW i-InGaAs—InP/p⁺InGaAs/n-InP, whichare formed on a semi-insulating InP substrate, are exemplarilydescribed. This invention may, however, be embodied in different formsand should not be construed as limited to the embodiments set forthherein. Rather, these embodiments are provided so that this disclosureis thorough and complete and fully conveys the scope of the invention tothose skilled in the art. In the drawings, the thicknesses of layers maybe exaggerated for clarity, and the same reference numerals are used todenote the same elements throughout the drawings.

FIG. 1 is a cross sectional view of an optoelectronic (OE) transmitterintegrated circuit (IC) according to an exemplary embodiment of thepresent invention.

Referring to FIG. 1, the OE transmitter integrated circuit includes anEA modulator and a DHBT, which are formed adjacent to each other on asemi-insulating InP substrate 101. The DHBT is disposed in a drivingcircuit that drives the EA modulator. More specifically, the OEtransmitter integrated circuit is structured in a single chip such thatthe thickness of an absorption layer of the EA modulator is greater thanthat of an insertion layer of the DHBT, which facilitates the transportof charges.

More specifically, the extinction characteristic of the EA modulatormostly depends on the structure of an MQW region that is an activeregion. Basically, the EA modulator has a p-i-n diode structure. Anintermediate layer refers to an intrinsic region and includes the MQWstructure. Since an electric field is mostly applied to the intrinsicregion, in the case of a typical electrode, the thicker thickness of theintrinsic region plays an important role in reducing the capacitance ofthe EA modulator that determines the maximum speed limit. The EAmodulator is based on the principle that the absorption coefficient of amaterial varies with an externally applied electric field owing to aquantum confined stark effect (QCSE). Accordingly, it is important toappropriately control the composition, thickness, and strain of aquantum well barrier such that the amount of absorption is as increasedas possible at a desired driving voltage.

Therefore, the EA modulator according to the present invention has thep-i-n structure, which is comprised of a p⁺InGaAs layer 104, an MQWi-InP/InGaAs quantum well absorption layer 105, and an n-InP layer 106.Here, the quantum well absorption layer 105 is formed using a selectiveMOCVD growth process to a greater thickness than that of a quantum wellinsertion layer of the DHBT disposed in the modulator driving circuit.Also, the p⁺InGaAs layer 104 is connected to a p-electrode 102, whereasthe n-InP layer 106 is connected to an n-electrode 107 by asub-collector layer 124. Further, the resultant structure is covered bypolymers 103 and 108 for the purpose of surface protection andelectrical connection; but the p-electrode 102 and the n-electrode 107are partially exposed.

Typically, the foregoing DHBT includes a quantum well layer interposedbetween a base layer and a collector layer (hereinafter, a “QW insertionlayer” or “insertion layer”). The quantum well insertion layer has anintermediate bandgap between the bandgaps of the base layer and thecollector layer in order to alleviate a phenomenon that a potentialbarrier caused by a bandgap difference between the base layer and thecollector layer prevents current from passing through the transistor.Thus, because the quantum well insertion layer with an intermediatebandgap makes the flow of current smooth between the base layer and thecollector layer, the DHBT including the quantum well insertion layer canimprove the efficiency of collector current and employ a high breakdownvoltage of InP collector.

Accordingly, the DHBT according to the present invention has an n-p-nstructure, which is comprised of an n-InP collector layer 123, an MQWi-InGaAs/InP quantum well insertion layer 120, a p⁺InGaAs base layer119, and an n-InP emitter layer 116, which are disposed over thesemi-insulating InP substrate 101. Here, the quantum well insertionlayer 120 is formed using a selective MOCVD growth process to a smallerthickness than that of the quantum well absorption layer 105 of the EAmodulator. Also, the n-InP collector layer 123 is connected to collectorelectrodes 109 and 122 by the n⁺InP sub-collector layer 124, thep⁺InGaAs base layer 119 is connected to base electrodes 111 and 117, andthe n-InP emitter layer 116 is connected to an emitter electrode 113 byan n⁺InGaAs ohmic layer 115. In addition, for the purpose of surfaceprotection and electrical connection, the n-InP collector layer 123, theMQW i-InGaAs/InP quantum well insertion layer 120, and the p⁺InGaAs baselayer 119 are covered by polymers 110 and 118, the n-InP emitter layer116 and the n⁺InGaAs ohmic layer 115 are covered by polymers 112 and114, and the n⁺InP sub-collector layer 124 is covered by the polymer 108and a polymer 121. In this case, the DEBT is formed adjacent to the EAmodulator on the same semi-insulating InP substrate 101.

As described above, in the present invention, the DHBT formed ofn-InP/p⁺InGaAs/MQW InP—InGaAs/n-InP is integrated as a single chip withthe waveguide type EA modulator, which includes the thick quantum wellphoto absorption layer obtained by the selective growth process, on thesame semi-insulating substrate. In this structure, the OE transmitterintegrated circuit chip can be used for high-speed opticalcommunications and obtain high modulation efficiency and a stabledriving characteristic.

FIGS. 2A and 2B are cross sectional views of a substrate before andafter a selective growth process is performed, and FIG. 3 is a crosssectional view of a substrate on which the selective growth process anda final growth process are performed.

At the outset, insulating masking patterns 304 as shown in FIG. 2A areused in order to grow crystalline layers having enhanced thicknesses onthe surface of a semi-insulating InP substrate 301. Referring to FIG.2A, an n⁺InP sub-collector layer 302, an n-InP layer 303, and twoinsulating patterns 304 are formed on the substrate 301. The insulatingpatterns 304, for example, each have a width of 30 μm and are spaced 20μm apart from each other. Specifically, when the IInGaAs—InP QW layersare grown on the i-InP layer 303 of the semi-insulating InP substrate301 on which the insulating patterns 304 are formed for a selectivemetal organic chemical vapor deposition (MOCVD) process, a QWcrystalline layer is grown between the two insulating patterns 304 andanother QW crystalline layer is grown outside a region between theinsulating patterns 304. The two crystalline layers have quantum wellstructures with different thicknesses of well and barrier and bandgapenergies according to the layer width and gap space of the insulatingpatterns 304. As a crystalline layer is grown to a greater thickness, alarger amount of light is absorbed into the cross section of thecrystalline layer and coupled with the crystalline layer. As a result,the light coupling and modulation efficiency is improved.

In other words, as shown FIG. 2B, by use of a selective MOCVD growthprocess, an absorption layer 305 a for an EA modulator and an insertionlayer 305 b for a DHBT are formed to respectively different thicknessesaccording to the width and gap of dielectric masks (i.e., the insulatingpatterns 304). Thus, the DHBT, which is formed of n-InP/p⁺InGaAs/MQWInP—InGaAs/n-InP, can have optimized characteristics, while theabsorption layer 305 a of the waveguide type EA modulator becomesthicker enough to enhance the light coupling and modulation efficiency.For instance, as each of the dielectric masks has a greater width, morenumber of element atoms move from the dielectric masks toward thesurface of the grown layer. Thus, the thickness of the grown QW layerbecomes thicker and the bandgap energy of its quantum well structurebecomes lower. Therefore, while the absorption layer 305 a of the EAmodulator and the insertion layer 305 b of the DEBT are being formedusing a single epitaxial growth process, the absorption layer 305 a canbe grown to a thicker thickness than that of the insertion layer 305 b.As a result, a larger amount of light couples to the absorption layer305 a of the modulator at certain modulated wavelength band, thusenhancing the modulation efficiency.

For example, as shown in FIG. 3, additional semiconductor layers 306,307, and 308 may be formed on the selectively grown quantum well layershaving different thicknesses, and then the EA modulator and the DHBT maybe fabricated. In this case of the MQW layers which act as theabsorption layer 305 a of the waveguide type EA modulator that operatein a certain wavelength of 1.55 μm, MQW insertion layer 305 b of theDHBT is grown to have an intermediate bandgap (about 0.9 eV in thewavelength of 1.45 μm) between the bandgaps of a base and a collectorusing the selective MOCVD growth process. Thus, the DHBT in which theflow of current is not blocked between the base and the collector can beintegrated in a single chip using a single epitaxial growth process. Inthis regard, according to the present invention, when the absorptionlayer 305 a of the waveguide type EA modulator is designed, the width ofthe insulating patterns 304 as masks can be optimized so that the EAmodulator can obtain high modulation efficiency at the desired lightwavelength.

Hereinafter, a method of fabricating the above-described OE transmitterintegrated circuit using a selective MOCVD growth process will bedescribed with reference to FIGS. 2A through 4F.

FIGS. 4A through 4F are cross sectional views illustrating a method ofan OE transmitter integrated circuit according to an exemplaryembodiment of the present invention.

At the outset, as shown in FIG. 2A, an n⁺InP layer 302 and an n⁻InPlayer 303 are sequentially grown on a semi-insulating InP substrate 301.Thereafter, insulating patterns 304 formed of an insulating material areformed on the n⁻InP layer 303, and a selective MOCVD process is carriedout using the insulating patterns 304 so that an absorption layer 305 afor a waveguide type EA modulator and an insertion layer 305 b for aDHBT are grown at the same time. Here, the insertion layer 305 b makesthe flow of current between the base layer and collector layer smooth inthe DHBT.

In this fabrication process, as shown in FIG. 2B, the absorption layer305 a for the waveguide type EA modulator is grown between theinsulating patterns 304 formed to a greater thickness than that of theinsertion layer 305 b, which facilitates the flow of current between thebase layer and collector layer in the DHBT. For example, when theinsulating patterns 304 are each formed to a patterned layer width ofabout 30 μm and are spaced about 20 μm apart from each other, theabsorption layer 305 a formed of MQW InP/InGaAs, which is grown betweenthe insulating patterns 304, is formed to a relatively thickerthickness, whereas the insertion layer 305 b of the DHBT, which is grownoutside a region between the insulating patterns 304, is formed to arelatively thinner thickness.

Therefore, according to the present invention, the OE transmitterintegrated circuit with high modulation efficiency and a stable drivingcircuit with high breakdown voltage of the DEBT characteristic can befabricated using a single selective epitaxial MOCVD process.

Thereafter, as shown in FIG. 3, after the absorption layer 305 a for thewaveguide type EA modulator and the insertion layer 305 b for the DHBTare grown, the insulating patterns 304 are removed, and a p⁺InGaAs baselayer 306, an n⁺InP emitter layer 307, and an n⁺In0.53Ga0.47As ohmiclayer 308 for improving an ohmic characteristic are grown. Thus, acrystalline structure for a single-chip OE transmitter, in which theabsorption layer 305 a for the EA modulator is grown to a greaterthickness than that of the insertion layer 305 b for the DHBT on thesame wafer plane, is completed.

Once the crystalline wafer structure for the OE transmitter with highmodulation efficiency is completed by growing the absorption layer 305 aof the waveguide type EA modulator to a greater thickness than that ofthe insertion layer 305 b for the DHBT on the same wafer plane, the DHBTis formed masking the EA modulator part by using an insulating layer(not shown) and an ordinary photolithography process. Referring to FIG.4A, first of all, the insulating layer is formed and etched using thephotolithography process. Thereafter, by using a reactive ion beametching (RIE) process, the n⁺In0.53Ga0.47As ohmic layer 308, the n⁺InPemitter layer 307, the p⁺InGaAs base layer 306, MQW InGaAs—InP layer305, the n-InP collector layer 303, the n⁺InP sub-collector layer 302,and a predetermined portion of the semi-insulating InP substrate 301between the absorption layer 305 a and the insertion layer 305 b areetched so that a region for the waveguide EA modulator and a region forthe DHBT are isolated from each other.

After the region for the waveguide type EA modulator is masked using theinsulating layer and the photolithography process, the DHBT isfabricated.

Specifically, as shown in FIG. 4B, the n⁺In0.53Ga0.47As ohmic layer 308and the n-InP layer 307 are etched using a selective etching process toform a mesa type emitter 307 a, and an emitter electrode 309 is formedon the ohmic layer 308.

Subsequently, as shown in FIG. 4D, the p⁺In0.53Ga0.47As base layer 306is exposed using a photolithography process and a lift-off process, anda Ti/Pt/Au layer is deposited, thereby forming base electrodes 310.

After that, as shown in FIG. 4E, the MQW InGaAs/InP insertion layer ofquantum well structure, which is inserted between the base layer 306 andthe collector layer 303 and allows the smooth flow of current betweenthe base layer 306 and the collector layer 303, and the n-InP collectorlayer 303 are selectively etched, and collector electrode pads areformed on a n⁺InP sub-collector layer 302 using a lift-off process. ATi/Pt/Au layer is deposited on the collector electrode pads, therebyforming collector electrodes 311. Thus, the DHBT is completed.

Thereafter, the EA modulator is fabricated. To begin, as shown in FIGS.4B and 4C, the n⁺In0.53Ga0.47As ohmic layer 308 and the n-InP layer 307,which cover the region for the EA modulator, are removed using anetching process. Then, by using the insulating layer as an etch mask,the p⁺InGaAs base layer 306, the MQW InGaAs—InP quantum well absorptionlayer 305 a, the n-InP layer 303, and the n⁺InP sub-collector layer 302are etched using photolithography and etching processes, so that thecrystalline structure of the EA modulator has a waveguide shape.Thereafter, a region for an electrode is prepared using photolithographyand lift-off processes, and a Ti/Pt/Au layer is deposited, therebyforming a p-electrode 312.

As shown in FIG. 4E, a photolithography process for a lift-off processis performed, and a Ti/Pt/Au layer is deposited, thereby forming ann-electrode 313. After that, a thermal anneal process is carried out sothat the EA modulator is completed.

Finally, as shown in FIG. 4F, a polymer 314 is formed to protect thesurfaces of the waveguide type EA modulator and DHBT and electricallyconnect the modulator and the DHBT, and an airbridge metal line (notshown) is formed using a lithography process between the n-electrode 313of the EA modulator and the output terminal of the collector electrode311 of the DHBT. Thereafter, a gold-plated layer 315 is formed on therespective electrodes 309, 310, 311, 312, and 313. As a result, asingle-chip OE transmitter integrated circuit in which the waveguidetype EA modulator and the DHBT formed of n-InP/p⁺InGaAs/MQWInP—InGaAs/n-InP are integrated is embodied.

In conclusion, according to the present invention, the OE transmitterintegrated circuit in which the waveguide type EA modulator isintegrated on a single chip can be fabricated using a selective MOCVDprocess. In the OE transmitter IC, the absorption layer of the waveguidetype EA modulator is formed to a greater thickness than that of thequantum well insertion layer of the DHBT, which improves the efficiencyof collector current, on the same wafer plane. That is, the waveguidetype EA modulator, which has high quantum and modulation efficiency anda high speed characteristic, and the DHBT, which is formed ofn-InP/p⁺InGaAs/MQW InP-InGaAs/n-InP and has an excellent amplificationcharacteristic, can be integrated on the same semi-insulating InPsubstrate. As a result, since the EA modulator has the high modulationefficiency and the DHBT has a stable driving characteristic caused by ahigh breakdown voltage, it is possible to fabricate the OE transmitterintegrated circuit with both excellent modulation efficiency andstability.

As explained thus far, according to the present invention, an EAmodulator with a relatively thick quantum well absorption layer and aDHBT with a relatively thin quantum well insertion layer are integratedon a single substrate, so that an OE transmitter integrated circuit withhigh modulation efficiency and a stable driving characteristic can befabricated. Also, optical fibers can be easily aligned to the EAmodulator with thicker absorption layer. Further, even though thewaveguide type EA modulator and the DHBT are integrated as a single chipusing a one-time selective growth process, they are independentlyoptimized in their performances. Therefore, the OE transmitterintegrated circuit can be fabricated using a simple process as comparedwith conventional single-chip fabrication that requires several growthprocesses.

Although exemplary embodiments of the present invention have beendescribed with reference to the attached drawings, the present inventionis not limited to these embodiments, and it should be appreciated tothose skilled in the art that a variety of modifications and changes canbe made without departing from the spirit and scope of the presentinvention.

1. An optoelectronic transmitter integrated circuit comprising: asemi-insulating substrate; an electroabsorption (EA) modulator disposedin a first region on the semi-insulating substrate and having a firstsub-collector layer, a first clad layer, a multi-quantum well (MQW)absorption layer, and a second clad layer; and a double heterojunctionbipolar transistor (DHBT) disposed in a second region adjacent to thefirst region on the semi-insulating substrate and having a secondsub-collector layer, a collector layer, an MQW insertion layer, a baselayer, an emitter layer, and an ohmic layer, wherein the MQW insertionhas a smaller thickness than that of the MQW absorption layer, whereinthe MQW insertion layer comprises an InGaAs MQW insertion layer, andwherein the EA modulator and the DHBT are integrated as a single chipusing a one time selective metal organic chemical vapor deposition(MOCVD) growth process for independently optimizing the EA modulator tohave a high modulation efficiency and the DHBT to have a stable drivingcharacteristic caused by a high voltage breakdown.
 2. The optoelectronictransmitter integrated circuit according to claim 1, wherein the EAmodulator further includes: a first electrode disposed on apredetermined region of the second clad layer; and a second electrodedisposed on a predetermined region of the first sub-collector layer. 3.The optoelectronic transmitter integrated circuit according to claim 1,wherein the DHBT further includes: an emitter electrode disposed on apredetermined region of the ohmic layer and having a mesa structure; abase electrode disposed on a predetermined region of the base layer; anda collector electrode disposed on a predetermined region of the secondsub-collector layer.